Because you have an href. Return false from the event handler to prevent the default action:
<a href="" id= "main_menu_item_contact" onclick="SelectMenuItem('contact'); return false;">Contact</a>The usual thing to do when no non-JS alternative is available is to link to # or javascript:void(0); like so:
<a href="#" id= "main_menu_item_contact" onclick="SelectMenuItem('contact'); return false;">Contact</a>However, in your case you should link directly to the contact page, or create a seperate contact page to link to here if none exists, and set that as the href so non-JS browsers have somewhere to go.